Circular and flat oval air ducts for ventilation, climate control, suction, chip and fiber transport are usually composed of straight air duct sections and shaped pieces. Straight air duct sections are currently manufactured for the most part as folded spiral-seam pipes, and less often as side-seam pipes. Folded spiral-seam pipes are folded from a sheet metal band whose edges are folded together. Since manufacturing is fully automatic and very quick, folded spiral-seam pipes are very reasonably priced. Side-seam pipes are mostly manufactured manually and are therefore somewhat more expensive. They are used primarily in cases where the spiral seam would interfere for various reasons. Shaped pieces (bends, T/pieces, branch-offs, transitions, etc.) are used between the straight duct sections.
Various types of connection are used to connect duct sections and shaped pieces to form a continuous air duct. So-called plug-in connections are used for smaller pipe cross sections, while larger pipe cross sections are connected primarily with connection flanges located at the ends of each duct section. In known butt joints of this type, the connection flanges are crimped, riveted, point-welded or attached with self-cutting screws to the ends of each air duct section. As required, the contact site between connection flange and duct wall must be sealed. This also requires an exact centering of the abutting duct sections.
Because of this, the manufacture of these known butt joints is rather complex. Annular connection flanges must be produced and attached at each duct section, as well as sealed and centered. For plug-in connections, the expenditure is somewhat smaller during manufacture, but greater during installation. The known air ducts also have significant problems with regard to sealing and interference with the air flow in the duct due to projections and projecting parts inside the duct, and also as a result of not being centered. Especially during suction and during the transport of dust, chips, fibers, etc., the sheet metal edges, gaps, and projections inside the duct result in accumulations that can build up and obstruct the duct. When transporting room air, such an accumulation is extremely undesirable for hygienic considerations, since an ideal growth substrate for bacteria and fungi develops in the mostly warm and frequently moist environment. It has therefore been recognized as absolutely necessary that air ducts for room air must be cleaned. To facilitate cleaning, all gaps, grooves, projections, and in particular projecting parts such as sheet metal screws, rivets, etc., must be avoided.
DE 25 53 720 A1, on which the preamble of claim 1 is based, describes a butt joint between two air duct sections in which the connection flanges are molded in one piece to the ends of the duct sections. They consist essentially of annular edges that project at a right angle from the ends of the channel sections and have outer edges which are angled away from each other by about 10.degree. in reference to the associated annular edge.
A clamp made of spring steel has been clamped to the annular edges at four points, offset from each other on the duct circumference by 90.degree.. The bent, outside edges of the annular edges prevent the clamp from slipping off the annular edges. The disadvantage hereby is that several elastic clamps must be manufactured and installed for a single butt joint. This type of installation does not allow a continuous, complete seal and a good centering of the air duct sections. The contact pressure that can be achieved in this manner between opposing annular edges is much too low for this. The stability of the assembly is also relatively small. Especially for pipes with larger diameters, e.g. about 2,000 mm, neither an adequate centering nor a stable suspension of the pipes can be achieved.